1. Field of the Invention
The present invention relates to an intake duct for introducing outside air into an internal combustion engine and more particularly to an intake duct which can reduce intake noise.
2. Related Art
In an intake system of an automotive engine, there exists a problem that intake noise is generated from an intake duct when outside air is introduced therethrough into the engine. This intake noise becomes noticeable in particular when the engine speed is low.
Then, as an intake duct which can reduce intake noise, there is known an intake duct in which an opening is provided in a duct wall of the intake duct in such a manner as to establish a communication between the inside and outside of an intake passageway, and an air-permeable member is provided in such manner as to close the opening so provided (for example, refer to JP-A-2002-106431.).
As is shown in FIG. 15, this intake duct includes a duct main body 82 which is formed into a hollow tubular shape having in an interior thereof an intake passageway 80 for introducing outside air into an internal combustion engine and which has in a duct wall an opening 81 which establishes a communication between the inside and outside of the intake passageway 80 and a porous sheet 83 as an air-permeable member which is provided in such a manner as to close the opening 81. The duct main body 82 is such that two divided bodies 84 which are formed of a thermoplastic resin are connected together, and the porous sheet 83 is insert molded in the divided body 84.
In this insert molding, molten thermoplastic resin is injected into a cavity in such a state that the porous sheet 83 is disposed within a predetermined mold, so that an outer edge of the porous sheet 83 is impregnated with the molten resin so injected. Then, the molten resin is cooled to set in such a state that the resin that has intruded into interstices of fibers which make up the porous sheet 83 and the fibers get entwined in each other, so as to form a joining portion 83a where the outer edge of the porous sheet 83 is impregnated with the resin, whereby the porous sheet 83 is joined to the divided body 84.
According to the intake duct in which the opening 81 formed in the duct wall is covered by the porous sheet 83 in this way, the generation of standing wave is suppressed, for example, due to part of acoustic wave generated within the intake passageway 80 passing through the porous sheet 83, whereby intake noise can be reduced.
In the conventional intake duct that has been described above, however, the porous sheet 83 having substantially the same thickness as that of the divided body 84 is joined to the divided body 84 which make up the duct main body 82 in such a state that facing sides of the porous sheet 83 and the divided body 84 butt at each other (in such a state that an outer peripheral surface of the porous sheet 83 and an inner peripheral surface of the divided body 84 which defines the opening 81 butt at each other). Due to this, the joining strength of the porous sheet 83 relative to the divided body 84 becomes deficient, and hence, there has been a risk of a drawback occurring that an outer edge of the porous sheet, for example, is turned up outwards.